Space Heating Using Off-Peak Electric Heat Storage

This invention relates to an off-peak electrical heating system, and more particularly to a heating system which utilizes heat that is generated and stored in a heat storage medium during the off-peak hours of electricity demand.

This heating system can be used to heat a conventional home in the same manner as a conventional natural gas or oil furnace, or electrical heating system.

The basic principle of the off-peak electrical heating system is the storage during off-peak electrical power periods of heat in a suitable heat storage material. The stored heat is then withdrawn from the storage material on a "heat-on-demand" basis, thus relieving the electrical power system of the heating load. Operation in this manner leads to more effective utilization of the generation, transmission and distribution facilities of the electrical power system.

Suitable materials for heat storage are inexpensive halogen salts such as sodium sulphate (Na.sub.2 SO.sub.4), sodium chloride (NaCl), potassium choloride (KCl), sodium chloride (CaCl.sub.2), or mixtures of these salts. These salts, or salt mixtures, are cast or molded into blocks. Binding agents such as silicates may be used.

Electrical heating elements, which are used in generating and transmitting heat to the salt, or salt mixture blocks, are embedded in the blocks while the blocks are being cast or molded, or are wrapped around the circumference of the blocks.

The surfaces of the blocks may be sealed to prevent the transmission of obnoxious odors into and out of the blocks. The electrical heating elements may be coated with a corrosion protective material to prevent corrosion of the heating element by the salt or salt mixture.

The heating system may be of a closed loop type wherein cold air to be heated is passed through a heat exchanger wherein the cold air is heated by an exchange of heat from a closed separate system of air that is heated by the heat stored in the blocks. The heating system may also be of an open loop type wherein the cold air to be heated is separated into two portions and one portion is heated by the heat stored in the blocks before it is joined with the other portion. An air flow control regulates the relative proportions of air thereby providing control over the temperature of the mixed heated air.

Transmission of electrical power to the blocks is controlled by an uninterruptible electronic timer and clock. The maximum amount of heat that can be stored in the blocks is controlled by a variable capacity control. A timer by-pass control permits a partial supply of electrical power to be drawn for heating when insufficient heat is stored in the blocks.

FIG. 1 is a graph showing heat content per unit volume as a function of temperature for various heat storage materials;

FIG. 2a is an elevation view of a typical heat storage element;

FIG. 2b is a side view of the element shown in FIG. 2a;

FIG. 2c is a sectional view taken along section A--A of FIG. 2a;

FIG. 3a is a diagrammetric representation of a closed loop heating system;

FIG. 3b is a diagrammetric representation of an open loop heating system;

FIG. 4 is a diagrammetric representation of the heating system including control circuitry.

Selection of a suitable heat storage material is an important feature of this invention. Suitable heat storage materials will have the ability to store (1) a maximum amount of heat within (2) a minimum volume over (3) a minimum temperature rise.

Countries like Canada, which experience very low climatic temperatures in the winter, for efficiency reasons cannot rely on heat storage materials which necessitate large and bulky storage units for sufficient capacity, or very high temperatures. Furthermore, the heat storage material must meet such practical requirements as low cost, ready availability and ease of handling.

Among the materials which meet these requirements, but which, as far as is known, have not been applied to systems that can heat an entire house, are a variety of salts, and mixtures of these salts. The density and thermal conductivity of these salts in their natural form is low, but is considerably improved if the salts are in the form of solid blocks. Solid blocks are most readily obtained by casting the molten salt or salt mixture.

Depending on the particular application, artificial binding agents can serve to strengthen the block structure. Silicates may be used as binding agents.

A graphic plotting of heat content per unit volume as a function of temperature for various salts provides a useful means of evaluating potential heat storage materials (See FIG. 1). The slope of the line indicates heat content per unit volume per degree of temperature. Materials having maximum heat capacity per unit of volume per unit of temperature can therefore readily be identified.

Maximum usable temperature of the heat storage material must also be considered from the standpoint of casting temperatures and maximum system design temperature. Each salt has a certain melting point, and eutectic mixtures of salts have lower melting points than the pure salts.

Salts, and mixtures of salts, which have very high melting points are not generally satisfactory because of the difficulty in casting or molding blocks from such salts and salt mixtures. On the other hand, salts, and mixtures of salts, which have low melting points are not generally satisfactory because they might melt during peak operating temperatures of the heating system.

For purposes of constructing a heat storage element 1, the salt, or salt mixture, is formed in blocks with an electrical heating element 2 embedded in the block. A typical heat storage element 1 consists of a salt casting which surrounds the electrical heating element. Depending on the corrosiveness of the salt and its electrical and thermal conductivity properties, the electrical heating element 2 may be coated with a protective material 3 prior to casting.

The heat storage elements are grouped together and insulated to provide a heat storage cabinet 4, the number of heat storage elements depending on the heat storage capacity that is required. This heat storage cabinet 4 is a part of the space heating system which may be of a closed loop (See FIG. 3a) or an open loop type (See FIG. 3b).

In the closed loop system, heated air is supplied from a heat storage cabinet 4 to a heat exchanger 5 by means of air flow induced by a circulating fan 6. The heat exchanger 5 transmits some of the heat from the air from the heat storage cabinet 4 to the cold air that is taken from the space to be heated. The air is circulated as in a conventional hot air heating system.

In the open loop system a controlled portion of the return air from the space being heated passes through the heat storage cabinet 4 and is then mixed with the remaining portion of the air to provide warm air to the space being heated at the required temperature.

One reason for choosing between the open loop and closed loop systems is whether the heat storage material tends to trap and subsequently release odours. Certain materials will trap odours as they cool and then release them during a warm-up period. Such materials would require a closed loop unit design to avoid odours being circulated throughout the space being heated, such as a house. Alternatively, the surface of the heat storage elements 1 can be sealed by encapsulation or a coating, if this results in a cheaper arrangement with equally satisfactory operation.

The electrical heating elements 2 in the blocks of the heat storage cabinet 4 should have sufficient capacity to provide for both home heating and stored heat during the off-peak hours of electric power supply, the stored heat capacity being determined by cycle of operation. For example, if 180 KWhr is the estimated maximum heat requirement for a 24 hour period and electricity is to be drawn for only 8 hours during the night, the heating elements 2 must have 180/8 = 221/2 KW capacity. 71/2 KW are required during the 8 hour period for home heating, representing 60 KWhr, the remaining 15 KW are required for heating the blocks to store 120 KWhr of heat for the 16 hour period when electricity is not being drawn. Electrical power in North America is usually supplied in 220V single phase and is generally available for residences. The service for residences that are adapted to use this off-peak heat storage system is similar to that for an existing electrically heated home, except for the control unit (See FIG. 4) which limits power supply for the heating system to designated off-peak hours.

This control unit, and others which are not found on conventional furnaces, are described in more detail below.

The uninterruptible timer 8 includes a timer 7 and serves the function of providing a control signal to supply power to the blocks for a predetermined interval every 24 hours. In order to provide the uninterruptible feature, in case of power failure the timer 8 is powered by a battery 9. The battery 9 is continuously trickle-charged 10 when power is available. The uninterruptible timer 8 circuitry employ CMOS integrated circuits to reduce power drain to the point where small, inexpensive, rechargeable batteries can be used.

Digital logic switching is employed to automatically switch from the usual 60 cps power frequency to an onboard cps oscillator/frequency divider. This feature eliminates the need for an electromechanical relay.

The implementation of a variable timer period either 8, 9, or 10 hours) is accomplished by decoding the seven-segment output pulses from the "minutes" digit on a digital alarm clock integrated circuit. One pulse every ten minutes is obtained and is then divided and counted to obtain the correct timer period.

This uninterruptible time 8 scheme provides higher accuracy than is possible with a resistor-capacitor discharge timer and, more importantly for this application, a much lower power drain than is possible with electromechanical timers.

This control 11 serves to set the maximum energy to be stored in the insulated heat storage cabinet 4 at the choice of the user. This feature is implemented by varying the maximum temperature at which power to the cabinet 4 is switched off.

The temperature is sensed by a thermocouple 12. An integrated circuit comparator 13 (shown as Capacity Selector in FIG. 4) compares the thermocouple output voltage supplied by the capacity selector 17 to a reference voltage. Once the thermocouple voltage reaches the reference voltage, a comparator output signal actuates circuitry to switch off the power to the cabinet 4. The reference voltage is adjusted by means of a multi-position switch connected to a resistive voltage divider.

This scheme has the advantage of high accuracy, the accuracy being limited only by the accuracy of the thermocouple.

This control 14 acts to limit the minimum temperature which the space to be heated, such as a house, may reach as a result of a power failure during the charging cycle, or other cause of insufficient heat. At a preset temperature, a fraction of the heating system capacity is energized independently of the setting of the uninterruptible timer 8.

Emergency heat supply may be activated by a thermal switch or a thermocouple output compared to a preset voltage. Either of these means results in a limited supply of electric heating to the heated space provided that the capacity selector unit is in a heating position (as opposed to "off" or "fan"). The supply is limited since a power failure might affect a sizable region and the electrical distribution system would not cope with full electric heating plus peak daytime loads. The electrical power supply is sufficient only to insure that freezing temperatures will not occur in the heated space under such conditions.

Two methods can be used to regulate the amount of proportion of air which flows through or past the heat storage cabinet 4.

One method is to regulate the degree of opening of gate 15 on the inlet of the heat storage cabinet.

The second method is to use an air deflector which regulates the proportion of air which flows through the heat storage cabinet 4 and the air which by-passes the heat storage cabinet. In the second method, gates fitted on the inlet and outlet of the heat storage cabinet 4 will be either completely open or completely closed, (depending on the state of thermostat 16 located in the heated space). A deflector mechanism replaces gate 15 shown in FIG. 4. Thermostat 16 operates a fan control which controls the fan blowing air through the system.

It will be appreciated that the foregoing description is directed to a preferred embodiment of the invention and that various technical non-inventive modifications and variations can be made to the invention to adapt the invention to various uses. It is to be understood, therefore, that such modifications and variations fall within the spirit and scope of the invention and the following claims to the invention.